This invention is related to microwave bandpass filters. In the design of bandpass filters, it is a common practice to couple a number of resonant sections to increase the selectivity of the filter. In microwave applications, the resonant sections are in the form of waveguides or dielectric resonators.
The dual-mode filter is a class of filters, in which the signal is excited in two orthogonal modes, as described by Atia and Williams in an article, "Narrow Bandpass Waveguide Filters" published in IEEE Transactions on Microwave Theory and Techniques, Vol.MTT-20, pages 258-265, April, 1972; and in another article "Dual Mode Canonical Waveguide Filter", published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25, pages 1021-1026, December 1977. The descriptions can also be found in U.S. Pat. Nos. 3,697,898, "Plural Cavity Bandpass Waveguide Filter", and 4,060,779, "Canonical Dual Mode Filter". This type of filter offers significant performance, size and mass advantages over conventional direct coupled cavity filters, in that the number of required resonant sections are reduced for the same selectivity.
These filters, however, require cross irises to provide couplings between resonators. Two factors make the use of irises difficult and expensive. First, the dimensions of the irises are determined on the basis of a small-aperture approximation, which is not accurate if the dimensions (i.e. width and length of the slot) are comparable to the operating wavelength. Therefore, trimming of the irises is inevitable in the course of tuning and testing. Second, the irises have to be produced (machined and silver-plated) to a high degree of precision which contribute significantly to the high cost of producing the filters.
Recently, realizations are introduced of canonical and longitudinal dual-mode dielectric resonator filters without irises, as described by Zaki et. al. in the paper, "Canonical and longitudinal dual mode dielectric resonator filters without irises", published in the IEEE Transactions on Microwave Theory and Techniques, Vol.MTT-35, pp. 1130-1135, December 1987. These realizations have the significant advantage of eliminating the most expensive part of the filters, i.e., the coupling irises, and replacing each iris with a simple length of the dielectric resonant enclosure and a pair of tuning screws. The dielectric resonators are coupled through evanescent (cutoff) fields existing in the section in the enclosure. Although extremely attractive from a production point of view, this realization has the disadvantage that the filter becomes excessively long, especially if small couplings are needed in the narrow-bandwidth filters.